1. Field of the Invention
The present invention relates, in general, to integrated circuits and, more particularly, to integrated circuits having voltage regulator circuits generating an internal power supply voltage from an external power supply voltage.
2. Relevant Background
Integrated circuits (ICs) comprise thousands or millions of individual devices interconnected to provide desired functionality. Significant effort is expended to improve processing techniques so as to reduce the size of each individual device in order to provide greater functionality on a given IC chip at reduced cost. In general, smaller geometry devices operate faster with less power than do larger geometry devices. As device geometries are reduced the breakdown voltages of the devices and the isolation that separates the devices decreases also.
Electronic systems usually comprise ICs manufactured with a variety of technologies. This has created a need for multiple power supply voltages to be supplied to a single printed circuit board to support the various types of devices on that board. For example, many complementary metal oxide semiconductor (CMOS) devices are still available that minimum drawn dimensions of more than 0.8 microns and require a power supply voltage of 5.0 volts. In contrast, state of the art ICs such as microprocessors and memory circuits have gate lengths in the order of 0.35 microns and require a power supply voltage of 3.3 volts or lower.
To ensure a broad market for a particular IC, it should be compatible with commonly available power supply voltages for other IC's. A practical solution to this disparity is to provide voltage regulator circuitry integrated with the low voltage ICs that decreases the higher voltage (e.g., 5.0 V in the above example) to the lower voltage required by the small geometry device (e.g., 3.3 V). Hence, it is necessary to regulate the externally supplied power supply voltage inside of each of the small geometry ICs.
A conventional on-chip voltage regulator is designed to generate a lower voltage than the external voltage. Typically, a transistor is coupled in series between the external voltage node and the internal voltage supply node. The conductivity of the transistor is modulated to drop the excess voltage across the transistor. To limit undesirable voltage ripple on the internal voltage supply node, the time constant of the regulator is desirably much longer than the internal cycle of the device. This prevents undesired voltage ripple within a cycle that can upset analog voltage levels. Because of this, the internal voltage supply node should be heavily filtered by coupling a large capacitor between the internal voltage supply node and ground. In practice, however, filter capacitors consume a great deal of chip area without adding functionality. Cost and chip size considerations dictate limiting the filter capacitor to more modest sizes.
The limited capacitor size reduces the charge storage capability of the regulator and makes it more sensitive to high current demand by downstream circuits and devices. An example of such circuitry are sense amplifiers in a dynamic random access memory (DRAM). In a typical DRAM circuit, one sense amplifier is supplied for each bit line pair in the device. For each sense amplifier, the stand-by (i.e., non-switching) state requires relatively little current. When activated, however, each sense amplifier may draw more than 1000 times its standby current. As used herein, the term "activated" means a state in which a circuit is drawing high current whereas "stand-by" means a state in which a circuit draws little current even though power is applied. Moreover, state of the art DRAM devices may have more than 1000 sense amplifiers activated simultaneously, resulting in very high current draw on the regulator. During high current demand, regulation can become poor and the chip's internal voltage levels can vary significantly. It would be desirable to bypass the internal regulator altogether during these high current demand operations.